激光熔凝铝锂合金铜偏析对析出相特征及腐蚀行为的作用机制

The demand for lowering the weight and enhancing the service performance of aircraft has made laser welding structures of Al-Li alloys one of the key development directions at present. Subjecting to rapid melting and solidification processes, grain-boundary copper segregation tends to occur in the laser welds of 2-series Al-Li alloys. However, systematic study on the influence of grain morphology on the copper segregation behavior is not sufficient, and, also, the effect of copper segregation on the features of precipitation phases and the localized corrosion behavior is not clear. In this project, the 2-series Al-Li alloys used for aircraft skin are selected as the research objects. The laser welds will be obtained via laser-remelting approach. The character of grain-boundary copper segregation in different grain areas will be investigated. Meanwhile, the effect of copper segregation on the type, morphology, and distribution feature of grain boundary precipitates will be analyzed to reveal the mechanism of the impact of copper segregation on the precipitation behavior of secondary phases. The morphology of precipitation free zone (PFZ) under copper segregation condition will be characterized. Furthermore, the electrochemical feature of the micro-zones in Al(PFZ)/precipitates/Cu ternary corrosion system will be analyzed. By obtaining the localized corrosion behavior of the laser welds, the function mechanism of the ternary corrosion system to the localized corrosion behavior of Al-Li alloy laser weld will be studied in detail. The results of this project will enrich the fundamental theories in microstructural manipulation and corrosion behavior of Al-Li alloy laser welds, and it will also provide theoretical basis for the application of laser welding structures of Al-Li alloys on aircrafts.

减轻飞机自重并提高服役性能的需求，使得铝锂合金激光焊接结构成为当前的重点发展方向之一。由于经历快速熔化和凝固过程，2系铝锂合金激光熔凝组织往往会发生晶界Cu偏析，但有关晶粒形态对Cu偏析行为影响的系统研究仍然欠缺，Cu偏析作用下的析出相特征及局部腐蚀行为尚不清楚。本课题以飞机蒙皮用2系铝锂合金为研究对象，采用激光重熔方法获得熔凝组织，系统研究不同晶粒形态区域的晶界Cu偏析特征，阐明Cu偏析对晶界析出相类型、形貌及分布特征的影响规律，揭示Cu偏析对晶界第二相析出行为的作用机制；分析Cu偏析作用下晶界附近无沉淀区（PFZ）的形貌特征，探究Al（PFZ）/析出相/Cu三元腐蚀体系的微区电化学腐蚀行为，获得激光熔凝组织的局部腐蚀特征，揭示三元腐蚀体系对激光熔凝组织局部腐蚀行为的作用机制。本研究将丰富铝锂合金激光焊接组织调控与腐蚀行为的基础理论，为铝锂合金激光焊接结构在飞机上的应用提供理论依据。

铝锂合金是实现飞机机身结构轻质高强的关键材料之一，焊接接头腐蚀问题是制约铝锂合金焊接结构发展与应用的关键因素。本课题详细研究了铝锂合金激光熔凝组织的元素偏析特征、时效析出行为及其对腐蚀和力学性能的影响。研究发现：1）铝锂合金激光熔凝组织上表面对称分为：熔池中心的柱状晶区、熔池边缘的等轴晶区、介于熔池中心和熔池边缘之间的柱状晶区。由熔池心部到熔池边缘，晶界Cu偏析程度逐渐升高。熔池的抗腐蚀能力随Cu偏析程度的增大而降低。2）熔凝组织晶界与枝晶界存在显著的Cu、Mn、Zn偏析带，155℃和175℃时效处理24 h未改变Cu、Mn、Zn的偏析特征，但发生了Mg、Zn共偏聚。熔凝态组织偏析带部位的金属间化合物主要是TB-Al7Cu4Li相（等轴晶区）和θ-Al2Cu相（柱状晶区），155℃时效后偏析带均以TB析出相为主，175℃时效后转变为以T1-Al2CuLi析出相为主。3）时效引起熔凝组织耐腐蚀性能降低，其中熔池边缘的等轴晶区对腐蚀最为敏感。时效温度由155℃升高至175℃，熔池边缘区域的点蚀加重。通过155℃和175℃时效处理可使硬度和抗拉强度提高，但是塑性降低。析出相数量增多、尺寸增大以及Li含量升高（θ→TB→T1）是抗腐蚀性能降低的主要原因。本课题揭示了铝锂合金激光焊接结构的微观组织及在航空服役环境中的腐蚀行为与机理，可为铝锂合金成分设计、微观组织调控及先进连接技术的发展提供理论指导和数据支撑。